Titanium-ceramic nanocomposites fabricated by the mechanical alloying process

Transcription

1 Materials Science-Poland, Vol. 26, No. 2, 2008 Titanium-ceramic nanocomposites fabricated by the mechanical alloying process K. NIESPODZIANA 1*, K. JURCZYK 2, M. JURCZYK 1 1 Poznań University of Technology, Institute of Materials Science and Engineering, pl. Skłodowskiej-Curie 5, Poznań, Poland 2 Poznań University of Medical Sciences, Department of Conservative Dentistry and Periodontology, ul. Bukowska 70, Poznań, Poland Wide use of titanium and its alloys as biomaterials stems from their low elastic moduli, good fatigue strength and better corrosion resistance compared to other metals and alloys used in medicine. However, they have poor tribological properties and a release of titanium alloy elements into surrounding tissues can cause eventual inflammation, failure and removal of an implant. For this reason, there is a great need for creating composite materials using ceramic particles to reinforce titanium which would give the possibility of optimizing mechanical and biological properties. In the present work Ti hydroxyapatite (HA, 3, 10 vol. %) and Ti SiO 2 (3, 10 vol. %) nanocomposites were fabricated by a combination of mechanical alloying (MA) and sintering processes. Mechanical properties and corrosion resistance of these composites were investigated by the Vicker hardness measurement and in vitro studies. The experimental results show that Ti 10 vol. % HA and Ti 10 vol. % SiO 2 nanocomposites have good corrosion resistance (I c = , E c = 0.48; I c = , E c = 0.45, respectively) in comparison with microcrystalline titanium (I c = , E c = 0.47). Vickers microhardness of the prepared nanocomposites is a few times higher than that of microcrystalline titanium. In conclusion, titanium ceramic nanocomposite is a suitable material for hard tissue replacement from the point of view of both mechanical and corrosion properties. Key words: titanium; bioceramics; nanocomposite; mechanical alloying 1. Introduction Titanium and titanium alloys are employed widely in biomedical and dental applications because of relatively low moduli of elasticity, low densities, high strength, good biocompatibility and corrosion resistance. Titanium and titanium alloys are generally regarded to have good biocompatibility and high corrosion resistance in vitro [1] although accumulation of titanium in tissues adjacent to the implant has been re- * Corresponding author, katarzyna.niespodziana@put.poznan.pl

2 342 K. NIESPODZIANA et al. ported signifying metal release and corrosion in vivo [2, 3]. Besides, titanium and titanium alloys have relatively poor tribological properties because of their low hardness [4]. In addition, metal implants may loose and even separate from surrounding tissues during implantation because of their low bonding strength [5 7]. Much better biomaterials for hard tissue replacement implants may be acquired by the preparation of titanium composites. The most commonly used materials are ceramics such as hydroxyapatites, silicas or bioglasses. Hydroxyapatite (HA, Ca 10 (PO 4 ) 6 (OH) 2 ), the basic mineral in natural bones, shows excellent biocompatibility with the bone. Besides, HA is bioactive and able to form strong chemical bonds with natural bone [8, 9]. Silica (SiO 2 ) is also a bioactive material with high corrosion resistance. Unfortunately, neither hydroxyapatite nor silica can be used for hard tissue replacement implants due to their poor mechanical properties [10]. Therefore, the connection of excellent biocompatibility and bioactivity of ceramics with good mechanical properties of titanium is considered to be a promising approach for fabrication of more perfect hard tissue replacement implants. There are two creative ways to meet this idea: use of ceramics as a coating or as a reinforcing phase in metallic-ceramic composites [11]. In general, the coating techniques became a common method, although they have some drawbacks: the toughness of ceramic is low; during the coating stage interfacial cracks can occur; the metal/ceramic adherence is low and can diminish in time [12]. For this reason, composite materials containing both ceramic and titanium are expected to have broad practical applications due to the fact that each material can compensate for the shortcomings of the other. To date, however, there are only few reports dealing with the microcomposite of ceramics and titanium manufactured by powder metallurgy [13 15]. The aim of this study was to develop new materials composed of Ti and hydroxyapatite or silica for biomedical applications. The Ti HA (3, 10 vol. %) and Ti SiO 2 (3, 10 vol. %) composites were prepared by a combination of mechanical alloying and powder metallurgical process. The structure and mechanical and corrosion properties of the composites were investigated. 2. Experimental The starting materials used were titanium (< 45 μm), hydroxyapatite and silica (< 68 μm) powders. The raw powders with various Ti/HA and Ti/SiO 2 mixing ratios were first blended by mechanical alloying processes under argon atmosphere using an SPEX 8000 Mixer Mill. The vial was loaded and unloaded in a Labmaster 130 glove box in high purity argon atmosphere. The mixture of Ti HA (3, 10 vol. %) and Ti SiO 2 (3, 10 vol. %) powders were ball milled for 44 and 20 h, respectively. Then the milled powders were compacted at 1300 MPa and heat treated at 1150 C in a gas atmosphere composed of 95% Ar and 5% H 2 under the pressure of 0.15 MPa holding for 2 h.

3 Titanium-ceramic nanocomposites 343 X-ray diffraction (XRD) was employed to study the effect of mechanical alloying and of any subsequent heat treatment on the phases present in the samples. Typical crystallite sizes were estimated from the half-width of lines using the Scherrer equation. A scanning electron microscope (SEM) with energy dispersive spectrometry (EDS) was used to study the microstructure and chemical composition of the produced samples. The effect of various amounts of HA or SiO 2 on mechanical properties of titanium composites was assessed by density measurements (Archimedes method) and Vickers microhardness tester on polished surface under the load of 200 g. The corrosion resistance in 0.1 M H 2 SO 4 aqueous solution was measured using in vitro potentiodynamic corrosion tests. Saturate calomel electrode was used as the reference electrode and graphite as the counter electrode. The polarization curves were obtained for each specimen and corrosion potentials and corrosion current densities were determined by the Taffel extrapolation methods. 3. Results and discussion MA process on Ti HA (3, 10 vol. %) and Ti SiO 2 (3, 10 vol. %) composites has been studied by X-ray diffraction. For example, Fig. 1a shows the XRD pattern of the starting titanium and hydroxyapaptite powders. During MA process, the originally sharp diffraction lines of starting materials gradually become broader and their intensities decreased with the milling time. The amorphous phase forms directly from the starting mixture of the compounds (Ti, HA) without formation of other phases (Fig. 1b). Formation of the nanocomposites was achieved by annealing of the amorphous material in high purity gas atmosphere composed of 95% Ar and 5% H 2 at 1150 ºC for 2 h. XRD analysis of Ti 3 vol. % HA showed the presence of α-ti (hexagonal-type structure with cell parameters a = Å, c = Å) and HA phases (Fig. 1c). When hydroxyapatite is added to titanium, the lattice constants of Ti decrease, as manifested by a shift of the diffraction peaks of the (100), (002) and (101) crystal planes of titanium towards larger angles in comparison with pure titanium. XRD analysis of Ti 3 vol. % SiO 2 showed the presence of α-ti (hexagonal-type structure with the cell parameters a = Å, c = Å). The formation of crystalline SiO 2 phase was not observed (Fig. 1d). Table 1 shows the average crystallite size of titanium-silica and titaniumhydroxyapatite composites estimated from XRD experiment. The results of the EDS analysis and a scanning electron micrograph of the surface of Ti 3 vol. % HA and Ti 3 vol. % SiO 2 nanocomposites mechanically alloyed and heat treated at 1150 C for 2 h are shown in Fig. 2. EDS analysis shows that the Ti 3 vol. % SiO 2 nanocomposite consists of titanium matrix with silica or silicon particles (Fig. 2a). A similar microstructure can be observed in Ti 10 vol. % SiO 2 nanocomposite.

4 344 K. NIESPODZIANA et al. Fig. 1. XDR spectra of Ti 3 vol. % HA and Ti 3 vol. % SiO 2 nanocomposites: a) Ti, HA powders 0 h MA, b) Ti 3 vol. % HA 44 h MA, c) Ti 3 vol. % HA after annealing, d) Ti 3 vol. % SiO 2 after MA and annealing Table 1. Characteristics of Ti HA and Ti SiO 2 nanocomposites and titanium Nanocomposite d [nm] HV 0.2 ρ [g/cm 3 ] Ti 3 vol. % HA Ti 10 vol % HA Ti 3 vol. % SiO Ti 10 vol. % SiO Ti The EDS results indicate that a predominant phase in Ti HA composites is titanium with some amount of apatites with various Ca/P ratios. The Ti 3 vol. % HA nanocomposite consisted of titanium matrix with apatites (Fig. 2b) with the Ca/P ratios 1.6 and 0.51 which were close to the values characteristic of hydroxyapatite and Ca(PO 3 ) 2, respectively. Ti 10 vol. % HA nanocomposite consisted of titanium matrix with calcium and titanium matrix with apatite, in which Ca/P ratio was 1.07, the value similar to the Ca/P ratio in Ca 2 P 2 O 7 (not shown). The existence of a second phase is a common feature of HA-based composites because the presence of titanium can degrade the structural stability of HA crystal and promote dehydration and decomposi-

5 Titanium-ceramic nanocomposites 345 tion of HA phase [15 17]. Besides, extreme heating conditions can produce secondary HA phases such as tricalcium phosphate (TCP), tetracalcium phosphate (TTCP) and calcium oxide (CaO). This is the result of cooling from a high temperature which can be postulated in the following reactions [18]: Ca 10 (PO 4 ) 6 (OH) 2 (HA)(s) 2Ca 3 (PO 4 ) 2 (TCP)(s) + Ca 4 P 2 O 9 (TTCP)(s) + H 2 O(g) Ca 4 P 2 O 9 (TTCP)(s) Ca 3 (PO 4 ) 2 (TCP)(s) + CaO(s) These secondary HA phases can influence the final properties of Ti HA nanocomposites as well. The presence of some amount of iron atoms in the nanocomposite Ti HA could be explained by Fe impurities trapped in the MA powders from erosion of the milling media [19]. Fig. 2. EDS spectra of surfaces: a) Ti 3 vol. % SiO 2, b) Ti 3 vol. % HA nanocomposites produced by mechanical alloying followed by annealing Properties of fabricated nanocomposites depend on the processing parameters. A detailed description of the influence of mechanical alloying and annealing times on mechanical properties of Ti 10 vol. % HA nanocomposite was given elsewhere [20, 21]. Differences in the phase contents of titanium-ceramic nanocomposites also

6 346 K. NIESPODZIANA et al. lead to changes of their properties. As shown in Fig. 3, Vickers microhardness of bulk samples prepared by mechanical alloying depends on the composite constitution. Vickers hardness of bulk samples prepared by mechanical alloying is higher than that of a pure microcrystalline Ti metal (250 HV 0.2 ) and strongly increases for Ti 10 vol. % HA nanocomposite (1500 HV 0.2 ) being three times higher than that of Ti 3 vol. % HA material (480 HV 0.2 ). Vickers hardnesses of Ti 3 vol. % SiO 2 and Ti 10 vol. % SiO 2 composites are 550 HV 0.2 and 670 HV 0.2, respectively. Fig. 3. The influence of volume fraction of HA or SiO 2 on Vickers hardness in titanium-ceramic nanocomposites Relative densities of the Ti HA composites are higher than 85% of the theoretical density of Ti and are lower than that of Ti SiO 2 composites (90% of theoretical density) (Table 1). The composites with lower volume fractions of ceramics have low densities, which indicates that they are more porous. Table 2. Mean values of corrosion current densities and corrosion potentials of Ti HA and Ti SiO 2 nanocomposites and titanium Composite I c [A/cm 2 ] E c [V] Ti 3 vol. % HA Ti 10 vol % HA Ti 3 vol. % SiO Ti 10 vol. % SiO Ti The corrosion current densities and corrosion potentials of various specimens were determined from the potentiodynamic polarization curves by the Tafel extrapolation method and summarized in Table 2. For example, the potentiodynamic polarization curves of the nanocrystalline Ti 10 vol. % HA and Ti 10 vol. % SiO 2 composites in 0.1 M H 2 SO 4 water solution are shown in Fig. 4. For the sake of comparison, the results obtained for microcrystalline titanium are also shown. The polarization curves

7 Titanium-ceramic nanocomposites 347 (Fig. 4b, c) of Ti 10 vol. % HA and Ti 10 vol. % SiO 2 composites were wider in the passive range in comparison with microcrystalline titanium (curve a). Fig. 4. Potentiodynamic polarization curves of: a) Ti, b) Ti 10 vol. % HA composites, c) Ti 10 vol. % SiO 2 in 0.1 M H 2 SO 4 distilled water solution According to Table 2, the corrosion resistance increases with the rise of ceramics contents. The corrosion resistance of the Ti 10 vol. % SiO 2 composite (I c = 9, A/cm 2, E c = 0.45 V) was better than Ti 3 vol. % SiO 2 composite (I c = 3, A/cm 2, E c = 0.42 V). The Ti 10 vol. % HA nanocomposite possesses higher corrosion resistance and thus higher corrosion current densities (I c = A/cm 2 ) than microcrystalline titanium (I c = A/cm 2 ) unlike Ti 3 vol. % HA composite (I c = A/cm 2 ). The Ti 3 vol. % HA nanocomposite executed worse corrosion properties in comparison with other composites and titanium because of its different chemical composition and low density, which indicates that it has a highly porous structure and could easier corrode in a corrosive environment. It is also seen that titanium composites with silica have better corrosion resistances than those with hydroxyapatite. Therefore, an addition of ceramics caused significant changes of corrosion behaviour of titanium. 4. Conclusions It can be concluded that the titanium-ceramic nanocomposites were successfully fabricated from Ti, HA and SiO 2 powders by mechanical alloying and powder metallurgical process. As the EDS analysis has shown, the Ti HA and Ti SiO 2 nanocomposites consist mainly of titanium rich matrix with various apatites and calcium or silica and silicon particles, respectively. The results show an improvement of the properties due to the nanoscale structures in consolidated materials. Vickers hardness changed upon density changes increasingd with the increase of ceramic content, especially in

8 348 K. NIESPODZIANA et al. Ti 10 vol. % HA composite. Besides, titanium-hydroxyapatite composite with 10 vol. % of HA possesses higher corrosion resistance than microcrystalline titanium. The corrosion resistance of the Ti 10 vol. % SiO 2 nanocomposite is also better than that of pure titanium. The results of electrochemical studies show that titaniumceramic composite with 3 vol. % of ceramic has lower corrosion resistance than microcrystalline titanium. As discussed above, Ti 10 vol. % HA or SiO 2 nanocomposite is suitable for heavy load-bearing hard tissue replacement from the point of view of both mechanical and corrosion properties. Acknowledgements The financial support of the Polish National Committee for Scientific Research is gratefully acknowledged (contract no N /2092). References [1] GONZALEZ J.E.G., MIRZA-ROSCA J.C., J. Electroanal. Chem., 471 (1999), 109. [2] MU Y., KOBAYASHI T., SUMITA M., YAMAMOTO A., HANAWA T., J. Biomem. Mater. Res., 49 (2000), 283. [3] BROWNE M., GREGSON P.J., Biomat., 21 (2000), 385. [4] LIU X., CHU P.K., DING C.H., Mater. Sci. Eng. R, 47 (2004), 49. [5] TAKESHITA F., AYUKAWA Y., IYAMA S., MURAI K., SUETSUGU T., J. Biomed. Mater. Res., 37 (1997), 235. [6] LONG M., RACK H.J., Biomater., 19 (1998), [7] KHAN M.A., WILLIAMS R.L., WILLIAMS D.F., Biomater., 20 (1999), 765. [8] HENCH L.L., J. Am. Ceram. Soc., 74 (1991), [9] AOKI H., Science and Medical Applications of hydroxyapatite, JAAS, Tokyo, [10] THIAN E.S., LOH N.H., KHOR K.A., TOR S.B., Biomater., 23 (2002), [11] HENCH L.L., Bioceramics, 81(1998), [12] TRENTZ OA., PLATZ A., HELMY N., TRENTZ O., Swiss Surg., 4 (1998), 203. [13] POPA C., SIMON V., VIDA-SIMITI I., BATIN G., CANDEA V., SIMON S., J. Mater. Sci. Mater. Med., 16 (2005), [14] CHENGLIN C., JINGCHUAN Z., ZHONGDA Y., SHIDONG W., Mater. Sci. Eng. A, 271 (1999), 95. [15] NING C.Q., ZHOU Y., Biomaterials, 23 (2002), [16] CHU C., LIN P., DONG X., XUE X., ZHU J., YIN Z., J. Mater. Sci.: Mater. Med., 13 (2002), 985. [17] GROSS K.A., BERNDT C.C., STEPHENS P., DINNEBIER R., J. Mater. Sci., 33 (1998), [18] TAS A.C., KORKUSUZ F., TIMUCIN M., AKKAS N., J. Mater. Sci.: Mater. Med., 8 (1997), 91. [19] JURCZYK M., SMARDZ K., RAJEWSKI W., SMARDZ L., Mater. Sci. Eng., A, 303 (2001), 70. [20] NIESPODZIANA K., JURCZYK K., JURCZYK M., Inż. Mater., 151 (2006), 636. [21] NIESPODZIANA K., JURCZYK K., JURCZYK M., Nanopages, 1, (2006), 219. Received 28 April 2007 Revised 16 February 2008